The invention relates to a process intended for dehydration and deacidizing and/or stripping and deacidizing of a natural gas, using a mixture of solvents and temperature control for deacidizing.
Treating a natural gas requires dehydration, stripping when the natural gas contains condensable hydrocarbons and deacidizing this gas when the proportion of acid gases contained therein is too high.
It is possible to dehydrate and to strip a gas such as a natural gas by cooling it in the presence of methanol so as to prevent formation of ice and/or hydrates.
It is well-known that, the gas being charged with methanol, a deacidizing stage can be carried out under advantageous conditions prior to the cooling stage by using for said deacidizing stage a mixture of solvents containing methanol.
It is also well-known that it is then possible to limit coabsorption of hydrocarbons by using a mixture of solvents comprising water, methanol and a solvent heavier than methanol.
The present invention allows, by reducing the temperature rise in the deacidizing column, to obtain excellent performances in terms of solvent flow rate and of acid gas absorption capacity.
Various heavy solvents can be used in the process according to the invention. The heavy solvent can be, for example, a polar solvent such as dimethylformamide (DMF), N-methylpyrrolidone (NMP) or dimethylsulfoxide (DMSO). The heavy solvent can also be a chemical solvent such as, for example, a secondary or tertiary amine, hydroxylated for example.
It is thus possible to combine the advantages of an amine as a chemical solvent and of methanol as a physical solvent. The presence of methanol notably allows to substantially reduce the proportion of solvent for relatively high acid gas contents in the gas to be treated. The presence of methanol also allows to absorb and to separate from the gas to be treated impurities such as, for example, mercaptans, carbonyl sulfide (COS) and carbon disulfide (CS2).
It is also possible, in the process according to the invention, to use fractions of solvent mixtures having different compositions to optimize the conditions of the gas scrubbing process performed by the mixture of solvents.
The process according to the invention can generally be defined as a process for treating a gas containing acid gases, wherein:
(a) at least a fraction of the gas is contacted with a methanol-containing aqueous phase, the gas being thus charged with methanol at the end of stage (a),
(b) the gas from stage (a) is contacted, in an absorption column, with a mixture of solvents comprising methanol, water and a solvent heavier than methanol, the gas flowing from the top of said absorption column being thus at least partly freed from the acid gases it contains at the beginning of the process prior to stage (a), the mixture of solvents flowing out at the bottom of said absorption column being charged with acid gases,
(c) the mixture of solvents from stage (b) is at least partly regenerated through pressure reduction and/or heating by releasing at least partly the acid gases, the at least partly regenerated mixture of solvents being recycled to stage (b) at the end of stage (c), and
(d) the gas from stage (b) is cooled by producing at least a methanol-containing aqueous phase which is at least partly recycled to stage (a), said process being characterized in that, in stage (b), the mixture of solvents flowing out at the bottom of said absorption column is cooled, then re-introduced at a higher level of said absorption column so as to control the temperature in said absorption column.
The process according to the invention is described in detail hereafter in connection with the diagram of FIG. 1.